The present invention relates generally to digital communications systems. More specifically, but without limitation thereto, the present invention relates to measuring the quality of a radio frequency channel used for routing message traffic in a digital communications system.
Traditionally, Internet Protocol (IP) based networks have been implemented with baseband communications systems; however, the recent convergence of the Internet Protocol space with broadband communications has resulted in a shift to communications networks that can support multiple Internet Protocol networks. The multiple Internet Protocol networks may be multiplexed onto the same physical medium, wherein each network utilizes a unique channel or set of channels. Although Internet Protocol network multiplexing may be performed in a variety of dimensions including time, space, and frequency, the most common multiplexing technique currently implemented in the broadband cable industry is frequency multiplexing.
An important aspect of Internet Protocol network multiplexing is the characterization of each channel supported by the physical medium for a communications system. Several characteristics may be measured to assess the quality of a channel. For example, for a coaxial cable, these characteristics include noise floor level, ingress noise characteristic, group delay (or phase distortion), micro-reflections, and amplitude distortion. These characteristics are often dependent on the specific path and direction of the transmitted signal. For example, the channel characteristics for a signal transmitted from node 1 to node 2 may differ significantly from those for the same signal transmitted from node 1 to node 3 or even from node 2 to node 1.
As the number of available channels in a network increases, it becomes even more beneficial to monitor channel quality and to adapt communications parameters to maximize network performance. The communications parameters include, for example, channel hopping, modulation adaptation, error correction, and equalizer parameters including tap values and depth. A preferred method for characterizing a channel is to transmit a reference signal through the physical medium along a selected path between the transmitter and the receiver and to measure the received signal. Generating an adequate reference signal for characterizing a channel often requires the capability to vary both the duration and the spectral content of the reference signal. This capability may be met by selecting the length and content of the message carried by the reference signal. A problem with this approach is developing a protocol for requesting and generating the reference signal for each different communications system data path.